[FEATURE] added ability to create default/fixed value nodes during XSD Schema Validation

[matthiashofele]

Description

This feature adds the functionality / ability to use defined default values from xsd schemas in xml config nodes.

This was probably not implemented due to backwards-compatibility to PHP 5.4
The constant LIBXML_SCHEMA_CREATE was introduced in Libxml >= 2.6.14 (as of PHP >= 5.5.2)

As of now magento 2 has a php requirement of 7.0.2|7.0.4|~7.0.6|~7.1.0 and it should be safe to use that extended functionality to harness the full power of XSD-Schemas.

Changes applied to \Magento\Framework\Config\Dom::validateDomDocument
Tests supplied in \Magento\Framework\Config\Test\Unit\DomTest::testValidateWithDefaultValue

See DOMDocument::schemaValidate

Fixed Issues

XSD schemas can define default values for xs:attribute nodes, but the xsd-based schema validation does not merge the defined default values into the merged XML \DomDocument

This behaviour is fixed by calling \DOMDocument->schemaValidate() with param LIBXML_SCHEMA_CREATE

[magento-engcom-team]

@matthiasherold thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[matthiashofele]

To be able to use this feature effectively in production mode the interface \Magento\Framework\Config\ValidationStateInterface which defaults to \Magento\Framework\App\Arguments\ValidationState should always return true (actually checks for developer mode).

The ValidationStateInterface is injected and used within the class \Magento\Framework\Config\Reader\Filesystem

This probably will slow down the caching-process a little (done in \Magento\Framework\Config\Data).
Once the caching is done the possibilty of being able to define default values for xml definitions far outweights the caching slowdown.

[magento-engcom-team]

Hi @slavvka, thank you for the review.
ENGCOM-3466 has been created to process this Pull Request

[ghost]

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